『Abstract
Unconformable surfaces separating Precambrian crystalline basement
and overlying Proterozoic to Cambrian sedimentary rocks provide
an exceptional opportunity to examine the role of primitive soil
ecosystems in weathering and resultant formation of saprolite
(weathered rock retaining rock structure) and regolith (weathered
rock without rock structure), but many appear to have been affected
by burial diagenesis and hydrothermal fluid flow, leading some
researchers to discount their suitability for such studies. We
examine one modern weathering profile (Cecil series), four Cambrian
paleoweathering profiles from the North American craton (Squaw
Creek, Franklin Mountains, Core SQ-8, and Core 4), one Neoproterozoic
profile (Sheigra), and one late Paleoproterozoic profile (Baraboo),
to test the hypothesis that these paleoweathering profiles do
provide evidence of primitive terrestrial weathering despite their
diagenetic and hydrothermal overprinting, especially additions
of potassium. we employ an integrated approach using (1) detailed
thin-section investigations to identify characteristic pedogenic
features associated with saprolitization and formation of well-drained
regoliths, (2) electron microprobe analysis to identify specific
weathered and new mineral phases, and (3) geochemical mass balance
techniques to characterize volume changes during weathering and
elemental gains and losses of major and minor elements relative
to the inferred parent materials. There is strong pedogenic evidence
of paleoweathering, such as clay illuviation, sepic-plasmic fabrics,
redoximorphic features, and dissolution and alteration of feldspars
and mafic minerals to kaolinite, gibbsite, and Fe oxides, as well
as geochemical evidence, such as whole-rock losses of Na, Ca,
Mg, Si, Sr, Fe, and Mn greater than in modern profiles. Evidence
of diagenesis includes net addition of K, Ba, and Rb determined
through geochemical mass balance, K-feldspar overgrowths in overlying
sandstone sections, and K-feldspars with reaction rims in weathered
basement. The sub-Cambrian paleoweathering profiles formed on
granite are remarkably similar to modern weathering profiles formed
on granite, in spite of overprinting by potassium diagenesis.』
Introduction
Important questions to be addressed.
Sample locations and background information.
Methods
Results
Macro- and micromorphological indicators of paleoweathering.
Petrographic indicators of diagenesis and hydrothermal alteration.
Discussion
Comparison with modern weathering profiles formed on granitic
parent materials.
Feldspar compositions of overlying Cambrian sandstone sections.
Most profiles are diagenetically or hydrothermally overprinted.
Conclusions
Acknowledgments
References cited